Author: Sietze Wijma

Categories
White wine

Sauvignon Blanc and its supposed cat pee aroma – The lowdown on 4-MMP

Introduction

One of the varietal aromatic compounds of Sauvignon Blanc is 4-MMP. In terms of its sensory descriptors it is almost like a chameleon: sometimes it smells like a box tree and on other occasions it will remind you of gooseberries. Some people indeed dare to say cat pee.

Origins

Flavour scientists have identified 4-MMP (full name: 4-mercapto-4-methyl-2-pentanone) as a constituent of the aroma of Sauvignon Blanc in as early as 1995 [1]. It is a varietal flavour classified as a primary flavour. This compound can be traced back to the grape, where it is present in a bound form. Bound flavour compounds are too heavy to volatilize and therefore cannot reach our nasal cavity, which implies they have no aroma or flavour. The bound form of 4-MMP is also referred to as the precursor. 4-MMP gets released during fermentation when yeast is able to break up the precursor.

Influencing factors – vineyard

Riper fruit has been associated with higher levels of the precursor of 4-MMP [2]. Moderate water insufficiency reduces 4-MMP. A high copper content in the soil, for example due to the use of Bordeaux mixture, may reduce levels of 4-MMP as well. It has been implied the copper binds to thiols.

Influencing factors – winery

Increased skin contact results in more extraction of the precursor to 4-MMP in the grape must. This technique is indeed sometimes applied in Sauvignon Blanc vinification. Fermentation is a key step to producing more 4-MMP, as the aromatic compound is released from its conjugate compound. Some commercial yeast strain are very efficient at breaking up the link between 4-MMP and the conjugate, resulting in higher levels of 4-MMP. See the illustration below of a commercial yeast strain.

Presence elsewhere

4-MMP is not the compound that gives cat urine is typical smell. A different yet very similarly structured compound is responsible for the smell of cat urine: 3-mercapto-3-methylbutan-1-ol. Both compounds are thiols.

3-mercapto-3-methylbutan-1-ol (cat urine)
4-mercapto-4-methyl-2-pentanone (Sauvignon Blanc)

Occasionally 4-MMP is found in other white grape varieties, such as Verdejo. 4-MMP can also be found in some red wines and is thought to contribute to black fruit characters in e.g. Cabernet Sauvignon. 4-MMP has also been identified in several varieties of American hops.

Descriptors

Box tree, cat urine, broom, underarm sweat, gooseberry, blackcurrant, elderflower.

Detection threshold

The detection threshold is the minimum quantity of the flavour compound required to be present to elicit a flavour sensation. For 4-MMP, this is about 1 nanogram / litre in wine. A nanogram (ng) is a million of a milligram (mg). One milligram is about the weight of sugar granule. We are looking at incredibly small quantities of flavour material here!

Less than 1% of the population is known to have specific anosmia towards this compound. Specific anosmia means someone is unable to smell a specific aromatic compound, while otherwise having an intact sense of smell.

Other origins

To the best of my knowledge, there are no other routes of 4-MMP to find its way into wine other than being a varietal character.

Chemical class

4-MMP is both a thiol and a ketone.

Thiols are sulphur-containing compounds that can give pleasant characteristics to wine in the case of 4-MMP, 3-mercaptohexanol (grapefruit), or coffee thiol (coffee). Other thiols may also play a role in minerality in wine.

Some thiols are associated with reduction and give undesirable flavour characters. An example is methanethiol (drains).

Ketones often give fruity or floral characters. Other examples of ketones in wine are β-ionone (violet) and β-damascenone (black tea)

Precursor

The precursor to 4-MMP is cys – 4-MMP. Cys is short for cysteine, the amino acid conjugate to which 4-MMP is bound. Yeast is primarily interested in the nutritional value of cysteine and therefore is equipped to break up the link between 4-MMP and cysteine.

Detection methods

Being a sulphur-containing compound, 4-MMP is rather volatile. It can be detected in wine by swirling the glass and taking short sniffs. 4-MMP is also quite strong on the palate. It may be more detectable by pinching your nose, taking some wine in your mouth, swallow/spit the wine then release your nose.

Some of the precursor to 4-MMP may have been left untouched by yeast during fermentation and is therefore present in the final wine. There is some evidence to suggests the precursor can break down in the mouth to release 4-MMP, due to enzymatic activity in your saliva [3]. This can result in a more pronounced sensation of 4-MMP on the palate.

Further reading

Lallemand yeast: https://catalogapp.lallemandwine.com/uploads/yeasts/docs/bff82035b08e2b009900a2208bfee36717c23bc2.pdf

[1]“. T. T. V. L. J.-N. B. a. D. D. Philippe Darriet, “Identification of a Powerful Aromatic Component of V vinifera L. var. Sauvignon Wines: 4-Mercapto-4-methylpentan-2-one,” 1995.
[2]W. J. d. T. Carien Coetzee, “A comprehensive review on Sauvignon blanc aroma with a focus on certain positive volatile thiols,” 2011.
[3]M. M. F. C. Sarah Ployon, “The role of saliva in aroma release and perception,” 2017.
Categories
White wine

Green bell pepper flavour in white wine

This flavour is neither the result of using green pepper extract, nor is it associated with soaking bell peppers in wine! So where does the often-mentioned aroma of green bell pepper come from?

Origins
It is widely accepted that the aromatic compound 2-isobutyl-3-methoxypyrazine is responsible for this flavour note. It is often abbreviated to IBMP and may go by the trivial name bell pepper pyrazine.
The compound is closely associated with Sauvignon Blanc and related grape varieties, and therefore considered a varietal flavour.
It is a primary flavour, i.e. a flavour originating from the grape or fermentation. The compound is found in the grape flesh and skins in its free, volatile form. Hence, when grapes contain IBMP, you would be able to notice it when tasting those grapes from the vine. Importantly, this is different from many other aroma compounds, including thiols and terpenes, which are bound to a grape component and hence odourless until released during fermentation.

Influencing factors – vineyard
Ripeness is a crucial factor. Bell pepper pyrazine peaks at veraison [2], after which it steadily declines. Vineyard temperature and sunlight exposure are also important factors. They lead to a decrease in IBMP. Training an open canopy can help lower levels of IBMP. Warmer sites, or those with slopes facing the sun typically yield grapes with lower levels of IBMP.

Influencing factors – winery
There are less option available in the winery to modulate the flavour. Juice clarification prior to fermentation reduces the compound to some extent. Oak ageing can also reduce the impact of the flavour by masking it with other flavours such as vanilla (from vanillin) and clove (from eugenol). Oak ageing, however, is not commonly practised with Sauvignon Blanc. Separate batches of wine with different levels of IBMP could be blended to modify the impact of IBMP. Other options in terms of winemaking are limited. IBMP is not very sensitive to oxidative treatments [3].

Presence elsewhere
Bell pepper pyrazine is the main constituent of the aroma of green bell peppers. In bell peppers, the compound develops in a similar way as in grapes. In green bell peppers, which are essentially the unripe version of its red counterpart, IBMP levels are high. It declines during ripening.

Descriptors
Common descriptors of wines high in IBMP include green, green bell pepper, vegetal, herbaceous, green beans, earthy, capsicum, jalapeños.

Detection threshold
In white wine, the detection threshold is about 4 nanogram / litre. That is very very little indeed. This detection threshold implies that at a concentration of 4 ng IBMP / litre wine, half of the population would notice the flavour. A nanogram equals one millionth of a milligram. To put that into perspective, a sugar granule is about 1 milligram.
Differences in sensitivity exist and the specific anosmia (=inability to smell specific compounds) rate of this compound is about 7% of the population. Don’t worry if you can’t pick up this particular compound – you may still be able to smell other compounds linked to Sauvignon Blanc, such as 4-MMP (smells of box tree or gooseberry). Other compounds linked to underripe grapes or stems include cis-3-hexenol (smells of freshly cut grass).

Other origins of methoxypyrazines
Inclusion of stems and leaves in the vinification process may increase levels of IBMP. IBMP and other methoxypyrazines are also secreted by ladybugs, causing ladybug taint in wine [4].

Other pyrazines in wine
IBMP is one of several methoxypyrazines found in wine.
Although methoxypyrazines are often shortened to simply ‘pyrazines’, they are not mutually exclusive. While all methoxypyrazines are indeed pyrazines, methoxypyrazines refer to the bell pepper pyrazine and potato pyrazine (2-isopropyl-3-methoxypyrazine). Potato pyrazine smells like potato skins or dug soil. In wine it is often at levels below the detection threshold, making this compound less relevant than its bell pepper equivalent.
Other pyrazines, not belonging to the methoxypyrazines family, are the result of the Maillard reaction, and can impart nuttiness and chocolate notes. Examples are 2,3,5-trimethylpyrazine (dark chocolate) and 2-acetyl pyrazine (popcorn). They may play a role in barrel ageing.

Detection methods
After swirling the wine, a short sniff of about one second should be enough to detect any methoxypyrazines present. The best way to familiarize yourself with the flavour is to smell a wine with a small quantity of bell pepper pyrazine added, and compare it with another glass with the same, untreated wine. There is another option: pour a Marlborough Sauvignon Blanc into a glass and pour a Chardonnay in a second glass. Sample the wines back and forth, and the green bell pepper character of the Sauvignon Blanc should become more apparent.

References

  1. Roujou de Boubee, D. (2003). Research on 2-methoxy-3-isobutylpyrazine in grapes and wine. Amorim Academy Competition, 21pp.
  2. Coetzee, C., & du Toit, W. J. (2012). A comprehensive review on Sauvignon blanc aroma with a focus on certain positive volatile thiols. Food Research International, 45(1), 287-298.
  3. Coetzee, C. (2011). Oxygen and sulphur dioxide additions to Sauvignon blanc: effect on must and wine composition (Doctoral dissertation, Stellenbosch: University of Stellenbosch).
  4. Pickering, G. J., & Botezatu, A. (2021). A Review of Ladybug Taint in Wine: Origins, Prevention, and Remediation. Molecules, 26(14), 4341.